1. Field of the Invention
This invention relates generally to a feed system for an antenna and, more particularly, to a feed system used in connection with a low observable, multifunction, conformal, load bearing structure, excitation notch antenna on an aircraft that provides radio communications and navigation functions over several frequency bands and communication modes.
2. Discussion of the Related Art
Modern manned and unmanned tactical aircraft require radio communications over several frequency bandwidths and communication modes to support the communications, navigation and identification (CNI) functions necessary for operation of the aircraft. These radio frequency bandwidths generally include the VHF frequency modulation (FM) band (30-88 MHz), the VHF amplitude modulation (AM) band (118-174 MHz) and the UHF band (225-400 MHz). To transmit and receive these frequency bands for aircraft communications purposes, a suitable antenna system is required that is positioned on the aircraft. Known antenna systems used on tactical aircraft for CNI functions have typically included blade antennas that have a fin protruding from the surface of the aircraft. Generally, multiple blade antennas are required for the CNI functions, including one for the VHF/FM frequency band, one for the VHF/AM frequency band and another one for the UHF frequency band.
Blade antennas have a number of drawbacks when used on aircraft. These drawbacks include requiring high speed electronics for synchronously tuning the antenna and the need to efficiently couple radio frequency (rf) currents to the aircraft skin. Coupling rf signals to the conductive portions of the aircraft is a known technique that is used when the only available antenna elements are electrically small in relation to the wavelengths of the signals being transmitted and received, as is the case for the VHF/FM band. Also, because the blade antenna protrudes from the aircraft, these antennas adversely effect the aircraft's aerodynamics. Additionally, broad band, electrically small, VHF/FM blade antennas have a very low gain because they have a poor matching network efficiency and a small radiation resistance.
Moreover, the available antenna installation sites on the aircraft may not support the number of antennas needed to install the proliferating number of CNI functions, if each function requires its own antenna. Excitation antennas for supporting CNI functions are installed on aircraft at sites that are not needed by other aircraft antennas and therefore provide an efficient use of the available "real estate" on the aircraft. Additionally, because many modern tactical aircraft are low observable aircraft, it is necessary that the antenna elements conform to the aircraft structure. Known conformal antennas do not cover all of the communications bands needed for the present CNI functions.
To overcome the requirement of multiple antennas to support the CNI functions, and eliminate the need for blade antennas, U.S. Pat. No. 5,825,332, filed Sep. 12, 1996, titled "A Multifunction Structurally Integrated VHF-UHF Aircraft Antenna System", assigned to the assignee of the instant application and herein incorporated by reference, discloses an aircraft antenna that is totally integrated within the aircraft, and operates over a wide range of frequencies, including VHF/FM, VHF/AM and UHF. To operate in this manner, the antenna system uses an electrically conductive element that is part of the aircraft structure and an antenna element positioned and shaped to form a notch therebetween. The notch is generally uniform in width over part of its length and flares to a larger width over the remainder of its length. Broad band impedance matching electronics are provided to couple the antenna system to a transceiver to provide efficient transfer of energy to and from the antenna.
The broad band conformal antenna system of the '332 patent discussed above includes an antenna feed connected to the matching electronics and to a selected antenna feed point on the antenna element. The antenna feed excites the antenna element for transmission of signals, and conducts the received signals from the antenna element and electrically conductive element of the aircraft structure. The feed structure in the '332 patent is a single piece structure that provides electrical coupling between the antenna element and the matching network, but is not effective to provide the necessary coupling and impedance matching over all of the desired frequency bands of interest, including VHF/FM, VHF/AM and UHF.
What is needed is an antenna feed structure that provides impedance matched feeds for all of the frequency bands for CNI functions in connection with a single multifunction antenna. It is therefore an object of the present invention to provide such a feed structure.